Optical communications systems as presently contemplated and constructed use a light source, such as a semiconductor laser or light emitting diode, which is optically coupled to a photodetector through a glass transmission line. The transmission line is commonly referred to as an "optical fiber." Either amplitude or frequency modulation may be used to convey information. If amplitude modulation is used, the information is transmitted as a bit stream of 1's and 0's with the bits being both transmitted and detected within predetermined time intervals.
Such systems presently operate at wavelengths between approximately 0.8 .mu.m and approximately 1.6 .mu.m with the longer wavelengths, that is, the wavelengths greater than appoximately 1.2 .mu.m, presently being considered most desirable for many systems applications because optical fibers presently used exhibit minimum loss and dispersion within this wavelength range. These characteristics facilitate design of optical communications systems having desirable properties such as high transmission rates, long distances between repeaters. etc.
However, to most efficiently utilize the minimum loss and dispersion properties of the optical fibers, the light source should not only emit radiation within this wavelength range but should also have its output concentrated in a narrow spectral range. In practical terms, this means that the light source should be a semiconductor laser emitting output of a single longitudinal mode. A single longitudinal mode is narrow enough in spectral width that it may be thought of as being a single frequency. The output contains unwanted secondary modes but these are greatly suppressed in intensity with respect to the wanted primary mode. The requirement that the output be a single longitudinal mode is easily understood by considering that a finite width pulse will spread, i.e., broaden, because of the dispersion properties of the fiber. If the spread becomes large enough, adjacent pulses will overlap and recovery of the transmitted information by the receiver will be impossible; i.e., the bits will not arrive at the photodetector within the proper time interval.
Accordingly, a variety of approaches has been taken in attempts to develop single longitudinal mode lasers. For example, distributed feedback (DFB) lasers have been developed. See, for example, Applied Physics Letters, 18, pp. 152-154, Feb. 15, 1971. Semiconductor lasers operating with an external cavity to produce single mode operation have been reported. See, for example, Electronics Letters, 18, pp. 1092-1094, Dec. 9, 1982. Additionally, coupled cavity lasers have been developed and have been shown to produce single longitudinal mode output even under high speed modulation. See for example, Electronics Letters, 18, pp. 901-902, Oct. 14, 1982, and Applied Physics Letters, 42, pp. 650-652, Apr. 15, 1983.
The operation of a coupled cavity laser is described in detail in the preceding references and may be briefly summarized as follows. A coupled cavity laser has two cavities which are optically coupled to each other. Each cavity has separate electrical contacts, i.e., the laser is a three-terminal device. One cavity operates as an oscillator while the other cavity acts as an etalon and provides mode selection. That the laser produces a single mode output is understood, for the case of gap widths approximately n.lambda./2, where n is an integer and .lambda. is the wavelength of the radiation, by considering that the mode of the coupled cavity laser must satisfy Fabry-Perot mode conditions in both cavities. More generally, modes are enhanced at local loss minima because of the constructive interference of optical energy reflected from the gap with that fed back from the other cavity, i.e., the etalon. This may occur near resonance or anti-resonance of the etalon as well as intermediate points. This is possible only for a limited number of discrete frequencies.
Variation of the etalon current permits tuning of the wavelength across the gain profile of the laser. However, a large number of mode hops may occur as the etalon current is increased from zero to values above threshold. Although stable output is observed between the mode hops, the mode hop boundaries, as a function of etalon current, vary with temperature and also perhaps with device aging. Thus, simply maintaining the etalon current at a constant value will not guarantee stable single mode operation. Mode hops are undesirable because they may cause information to be lost because, for example, bits are not transmitted as desired or they are lost because they are not received within the proper time interval. Thus, techniques to maintain single mode operation at a desired wavelength are desirable.
Wavelength stabilization techniques have been previously developed for both coupled cavity lasers as well as various other types of lasers such as semiconductor lasers operating with an external cavity. A common element of these prior art schemes is the requirement of an external optical element, such as either a photodetector or a spectrometer, to monitor the light output from, e.g., the laser. Feedback loops are then used to adjust, for example, the laser current or a property of the external cavity such as its length if it is a passive cavity.